The invention relates to a multi-stage counter-current crystallization apparatus for crystallizing and subsequently separating a component in pure form from a solution.
An apparatus for separating a component from a solution, also referred to by the common term suspension crystallization apparatus, is used both in the foodstuffs industry and in the chemical/petrochemical processing industry.
In the foodstuffs industry, the solution is usually an aqueous solution, such as milk or fruit juices, in which water is the component to be separated and in which the final concentrate, for example milk concentrate, is processed further as an intermediate product. In the case of aqueous waste flows, water is likewise extracted from the solution by means of suspension crystallization, after which the concentrate that is finally obtained can simply be incinerated as a waste product. In the chemical/petrochemical industry this technique is used for obtaining a particular component in pure form from a chemical mixture, for example for the recovery of pure para-xylene from a mixture of isomers.
In such a crystallization apparatus, a slurry or suspension, also called crystal suspension mixture is formed, consisting of crystals (the component to be separated) and a concentrated solution, also called mother liquor solution.
A crystallization apparatus as described above is known, for example from European patent publication No. 0 051 340. An important parameter in industrial suspension crystallization processes is the crystal or ice fraction of the crystal suspension mixture that is present in large, agitated crystal-growing vessels. In order to produce (preferably large) crystals in the best possible way, which is important in connection with an optimum utilisation of the capacity of the separating means, a long residence time of the crystals in the crystal-growing vessel is required so as to obtain the highest possible crystal fraction in the crystal-growing vessel. With a view to obtaining an optimum and maximum yield of crystals, the ice crystals must be present in the crystal-growing vessel for a sufficiently long period of time, so as to enable them to grow into large ice crystals which are thus easy to separate.
Usually, such suspension crystallization apparatuses are composed of several concentration stages, so that the highest possible concentrated mother liquor is solution can be obtained in the final stage, whilst the separation of the crystals from the solution can be carried out at a lower concentration. The latter is important because the separation proceeds more easily as the concentration and thus the viscosity of the solution is lower.
In European patent publication No. 0 051 340 a filter is arranged in the crystal-growing vessel, so that only mother liquor solution is carried from the crystal-growing vessel to the crystallization means consisting of a heat exchanger of the scraped surface type. The object of this is to convert the small crystals from the heat exchanger into large, round crystals by allowing them to mature in the crystal-growing vessel rather than have the growth of said-crystals take place through circulation past the heat exchanger. Said filter, however, limits the maximally obtainable crystal fraction in the crystal-growing vessel.
The presence of the filter in the crystal-growing vessel does not permit a maximum crystal fraction, because this leads to accumulation of crystals and thus to clogging of the filter by the crystals and any other non-dissolved particles that are present. Since such a clogged filter cannot be cleaned, whether or not by means of complicated cleaning means, and must be kept free from crystals, during operation of the crystallization apparatus, a clogged filter in the crystal-growing vessel will inevitably lead to the apparatus being put out of operation, which is undesirable.
Since the filter in the crystal-growing vessel can also lead to a high pressure drop, the crystal-growing vessel must be in the form of a pressure vessel, which makes the purchase of the crystal-growing vessel in particular and indirectly also of the apparatus in its entirety a costly affair. In addition to that, the filters require a great deal of maintenance.
It is an object of the present invention to obviate the drawbacks of the use of a filter in the crystal-growing vessel, to which end the crystal-growing vessel of at least one concentration stage is provided with a bypass conduit arranged outside the crystal-growing vessel for circulating the crystal suspension mixture that is present in the crystal-growing vessel, and wherein filter means are present in the bypass conduit for extracting at least part of the mother liquor solution from the crystal suspension mixture.
Surprisingly it has been demonstrated that although this arrangement according to the invention leads to more irregularly shaped crystals, this shape of the crystals does not have any negative effects on the capacity and effectiveness of the crystal separating means if the conditions are further identical.
More in particular, according to the invention the crystal-free mother liquor solution can function as the supply solution for a further concentration stage.
More in particular, the filter means consist of a pressure vessel arranged in the bypass conduit, which is provided with a mixing element and a filter, which filter can be cleaned at its crystal suspension side. The filter cleaning means may consist of a scraping element which Is movable relative to the filter surface.
In addition to that, a circulation pump may be arranged in the bypass conduit.
The invention furthermore relates to an apparatus for separating crystals from a crystal suspension mixture composed of crystals and mother liquor solution, comprising
a fully closed cylinder and a piston which is movable therein, which piston bounds a washing column chamber inside the cylinder;
at least one piston rod provided with a longitudinal bore, via which piston rod the piston is moved reciprocatingly in the cylinder by driving means;
filter means connected to the piston at the side of the washing column chamber;
a supply conduit, which is connected to the end of the piston rod that projects outside the cylinder, which conduit functions to supply the crystal suspension mixture to the washing column chamber through the longitudinal bore;
a discharge conduit for discharging mother liquor solution from the washing column chamber after filtration through the filter;
means arranged in the washing column chamber, near the closed side of the cylinder, for the controlled removal of an amount of crystals from the crystal bed that has been compacted in the washing column chamber by the piston;
a circulation conduit arranged outside the cylinder, which is in communication with the washing column chamber via a connection for the discharge of the removed amount of crystals and via a connection for the supply of a washing liquid to the compacted crystal bed; and
a conduit connected to the circulation conduit for discharging the removed crystals, which may or may not be in melted form.
Such a separating apparatus, which is also referred to as a washing column apparatus, is disclosed in, for examples U.S. Pat. No. 4,475,355. During operation, the crystal suspension mixture that is present in the washing column chamber is compressed by the piston, so that the crystals are compacted and carried to a circulation conduit by the removing means, for example a rotary disc fitted with scraping knives.
The circulation conduit, also referred to as the washing circuit, not only discharges the removed or scraped-off, compacted crystals from the washing column chamber, but it also supplies a washing liquid to the washing column chamber for washing the compacted crystal bed. Said washing liquid, in combination with a pressure exerted on the crystal bed by the piston, leads to the formation of an separation interface or wash front in the compacted crystal bed between the part of the compacted crystal bed near the removing means, in which the washing liquid has displaced the mother liquor solution that is present between the crystals, and the part of the compacted crystal bed near the piston or the filter means, in which the mother liquor solution is still present between the crystals.
The crystal bed that has been washed by the washing liquid can easily be carried off from the washing column apparatus with the aid of the removing means in order to obtain a pure component. The washing column apparatus thus effects the separation between the crystals and the mother liquor solution, which solution is carried to the crystal-growing vessel via the discharge conduit for further thickening or concentration.
According to the prior art, an adequate washing action requires the exertion of a high pressure by the piston on the compacted crystal bed that is present in the washing column chamber in order to build up a sufficiently high differential liquid pressure across the crystal bed, which leads to the desired flow of washing liquid towards the piston and the filter means for the purpose of displacing the mother liquor solution from the crystal bed.
Upon compacting of the crystal bed, the compacted crystal bed experiences a considerable amount of friction with the inner wall of the cylinder. In particular in the case of organic crystals and, to a lesser extent, ice crystals, said friction may become so high that further compacting of the bed by the piston is no longer possible. In such situations, the washing pressure to be built up by the piston is not high enough for displacing the mother liquor solution from the compacted crystal bed to a sufficient extent.
It is an object of the invention to obviate the aforesaid drawback and to provide a washing column apparatus by means of which a compacted bed of solid matter can be washed more efficiently. To this end, means for creating a differential liquid pressure across the compacted crystal bed during operation are arranged in the circulation conduit.
These additional pressure means extend the usability and the operating range of the washing column apparatus considerably, since the washing column can be operated at a higher washing pressure. Accordingly, the yield of solid matter increases considerably.
Specific embodiments of such a washing column apparatus are according to the invention characterized in that the pressure means consist of a restriction valve, which is incorporated in the circulation conduit between the discharge connection and the discharge conduit, or in that the pressure means consist of an external pump.
The pressure means may comprise an expansion vessel connected to the circulation conduit for creating the required differential liquid pressure by means of air pressure.
In another embodiment of the washing column apparatus according to the invention, the pressure means are controllable, more in particular by means of a sensor which detects the position of the separation interface between the part of the crystal bed in which the washing liquid has displaced the mother liquor solution that is present between the crystals and the part of the crystal bed in which the mother liquor solution is still present between the crystals.
In another embodiment, the supply conduit for supplying the crystal suspension mixture is connected to the longitudinal bore of the piston rod by means of a flexible intermediate conduit.